1. Field of the Invention
The present invention relates to cardiac event detection. More particularly, the present invention relates to a lower power, active band-pass filter circuit, and filtration method for use within an implantable cardiac pacer.
The major pumping chambers in the human heart are the left and right ventricles. The simultaneous physical contraction of the myocardial tissue in these chambers expels blood into the aorta and the pulmonary artery. Blood enters the ventricles from smaller antechambers called the left and right atria which contract about 100 milliseconds (ms) before the ventricles. The physical contractions of the muscle tissue result from the depolarization of such tissue, which depolarization is induced by a wave of spontaneous electrical excitation which begins in the sinus node of the right atrium, spreads to the left atrium and then enters what is known as the AV node which delays its passage to the ventricles via the so-called bundle of His. The frequency of the waves of excitation is normally regulated metabolically by the sinus node. The atrial rate is thus referred to as the sinus rate or sinus rhythm of the heart.
Electrical signals corresponding to the depolarization of the myocardial muscle tissue appear in the patient's electrocardiogram. A brief low amplitude signal, commonly referred to as the P-wave, accompanies atrial depolarization. The P-wave is normally followed by a much larger amplitude signal, known as the QRS complex, with a predominant R-wave signifying ventricular depolarization. Repolarization prior to the next contraction is marked by a broad waveform in the electrocardiogram known as the T-wave.
A typical implanted cardiac pacer (or pacemaker) operates by supplying missing stimulation pulses through an electrode on a pacing lead in contact with the atrial or ventricular muscle tissue. The electrical stimulus independently initiates depolarization of the myocardial (atrial or ventricular) tissue resulting in the desired contraction. The P-wave or R-wave can be sensed through the same lead, i.e., the pacing lead, and used as a timing signal to synchronize or inhibit stimulation pulses in relation to spontaneous (natural or intrinsic) cardiac activity. The sensed signals are referred to as an atrial electrogram or ventricular electrogram respectively.
Every modern-day implantable pacemaker includes a sensing circuit, whether the activity of one or both chambers of the heart are sensed. When the electrical signal of the atrial or ventricular electrogram is transmitted from the heart into the pacemaker, the electrical signal is typically passed through an amplifier, referred to as the sense amplifier. The sense amplifier serves two primary functions: (1) it amplifies the incoming electrogram electrical signal so that such signal (which may only be 0.1 to 10 millivolts (mV) when received) may be processed and detected more accurately; and (2) it filters the incoming electrogram signal to eliminate "noise" and other unwanted components that may be contained therein, e.g., the low frequency T-wave. Noise is commonly introduced into the electrogram signal from sources outside the body, e.g., florescent lighting, or inside the body, e.g., electrical signals in the muscles of the chest.
Typically, the sense amplifier is able to substantially filter out such noise and other unwanted components so as to provide a relatively "clean" amplified electrical signal to the other electronic circuits, e.g., a window detector and/or an analog-to-digital (A/D) converter, of the cardiac pacer. Unfortunately, however, the typical band-pass filter circuits used in cardiac pacing applications require two operational amplifiers, one for a high-pass stage and the other for a low-pass stage. Such two stage filters provide second order filtration and achieve generally favorable results. However, the amount of power consumed by these two stage filters is disadvantageously high, thereby increasing the demand on the implanted pacer's battery and, as a result, decreasing the battery's lifespan.
The internal circuits of an implantable pacer are typically integrated into a single integrated circuit or "chip." These integrated circuits have the advantage of being much smaller than similar circuits constructed either partially or completely from discrete components. The sense amplifier is one of the most important parts of such integrated circuit because for the pacemaker to operate properly, it must be able to accurately and reliably sense the incoming electrogram signal. For dual chamber pacemakers, two sense amplifiers on the integrated circuit are required, one for processing the incoming electrogram signal from the atrium, and the other for processing the incoming electrogram signal from the ventricle. At least one of these sense amplifiers should stay alert all of the time to continuously monitor the heart for the occurrence of electrogram signals. Hence, the power consumption of the sense amplifier that must be alert all of the time becomes a critical design parameter that can significantly affect the life of the pacemaker battery.
Further, because sense amplifiers must monitor, amplify, filter, and process electrogram electrical signals that are very small is magnitude (on the order of 0.1-10 mV), such sense amplifiers must be low noise circuits, meaning that any noise generated or picked up by such sense amplifiers should be less than about 0.1 mV. Disadvantageously, if the sense amplifier is not a low noise circuit, then the noise from the sense amplifier corrupts the incoming electrogram signal and makes it useless for further processing.
In theory, the realization of the band-pass filter function within a sense amplifier may be achieved in various ways. First, a passive filter, i.e., a filter comprising only passive components (such as resistors and capacitors) may be utilized. Advantageously, a passive filter is very low noise (i.e., it generates noise components having a magnitude on the order of only 10 .mu.v to 100 .mu.v). However, a passive filter circuit is simply not acceptable for use within a pacemaker because it does not amplify the signal. To the contrary, a passive filter attenuates the incoming signal.
Second, an active band-pass filter made from discrete components could be utilized. While discrete active band-pass filter circuits may be designed as very low noise circuits, such circuits are also not acceptable for use within a pacemaker. Their power consumption is too high, and they occupy too much space. Further, because they must be made from components mounted on a hybrid substrate, their use adversely impacts the reliability and manufacturability of the pacemaker.
Third, an active band-pass filter circuit made as an integrated part of the pacemaker's circuit chip may be utilized. Such approach is the preferred approach for implantable pacemakers because it occupies less space (area) on the integrated circuit chip, and consumes less power. However, because such circuits share a common substrate with all of the other circuits within the pacemaker, they tend to be somewhat noisier than other active filter circuits.
Therefore, improvements are needed in the filter circuits used in sense amplifiers of cardiac pacers.